How do you keep managers active once your league starts and what do you do if they don't?

This is my first season as commish in a basketball league. We expanded our team at the last minute from 4 to 16 to get more competition and opened it up to the public. The problem we're having is a few of our "managers" don't keep their rosters current which is making for easy wins when someone plays them. What would you do if you were in my "shoes"